Optical communication system having a liquid crystal routing switch

ABSTRACT

A twisted nematic liquid crystal-based electro-optic modulator with a twist angle between 0° and 90°, and preferably between 50° and 80° is provided. The modulator provides a relatively rapid switching time such as less than about 50 milliseconds, and provides relatively large extinction ratios, such as greater than -25 dB. Preferably the liquid crystal entrance director differs from the polarization direction by a beta angle of about 15°.

This application is a Divisional of application U.S. Ser. No.09/003,567, filed Jan. 6, 1998, now U.S. Pat. No. 6,094,246.

The invention generally relates to liquid crystal devices and, inparticular, spatial light modulators using twisted nematic liquidcrystal materials with acute twist angles for optical communication.

BACKGROUND INFORMATION

Much of the previous work involving liquid crystal (LC) devices has beendirected to display devices such as flat-panel displays. While asubstantial amount of process has been made in this regard, the lightmodulators and other devices associated with liquid crystal displaydevices do not necessarily serve to solve problems in connection withoptical communication applications, with which the present invention isprimarily concerned. For example, although reduction in switching timehas some usefulness in connection with liquid crystal display devices,this factor is of significantly greater importance in opticalcommunication systems. The standard known as the synchronous opticalnetwork (SONET) for fiber optics communications specifies that whenthere is a network interruption, recovery time should be less than 50milliseconds. Thus, in order to keep a SONET System in operation,optical switches should respond within 50 milliseconds.

One type of previous liquid crystal modulator having relatively rapidswitching time is that generally described as a parallel oranti-parallel nematic liquid crystal modulator. However, parallel (orzero degree) and anti-parallel nematic LC modulators have relativelypoor contrast (low extinctions ratios). In particular, due to theunidirectional molecular tilt at the cell boundaries, there issignificant residual birefringence when the electrical field is appliedto the cell which degrades the extinction ratio of the device. Foroptical communication applications, it is desired to achieve a contrastratio or extinction coefficient of at least about −25 dB, morepreferably about −30 dB, and even more preferably greater than −30 dBand up to −40 dB or more dB. Although the extinction ratio for parallelLC modulators can be improved e.g. by placement of a compensatingbirefringent polymer, this increases the complexity and, in most cases,the cost of the modulator (and reliability).

Another liquid crystal structure is known which provides relatively highextinction ratios by using a 90 degree twisted nematic (TN) modulator.Twisted nematic (TN) liquid crystal (LC) has been widely used inelectro-optic modulators in applications such as flat panel displays,spatial light modulators, and specialized optical image processors. Suchdevices are generally fabricated to define twist angles of 90° (forconventional twisting TN) or 180°-270° (for “supertwist” nematic (STN)structure). In this context the twist angle is the angle between thedirection of the entrance director and that of the exit director.

A typical TN-LC modulator is made by the following process. Transparentelectrode indium-tin-oxide coated glass substrates are generally usedfor the cell walls. They are spin-coated with alignment material, suchas nylon or polyimide, and then buffed, such as by rubbing with silk todefine a rubbing direction for each substrate, forming LC directions.The two substrates are brought together with the rubbing directions at90° with respect to one another (where the angles are measured in thesame sense as the LC material twist, i.e. calculated in a right-handedmanner when the modulator uses an LC material with right-hand twistcharacteristics and calculated in a left-handed manner when themodulator uses an LC material with left-hand twist characteristics).Liquid crystal molecules between the substrates are switched between twostates when an electrical field is applied to the electrodes. Thethickness of the liquid crystal cell is designed such that:$\frac{\Delta \quad n\quad d}{\alpha} = \frac{\lambda}{2}$

where Δn and d are the optical birefringence and the thickness of theliquid crystal material, and λ is the operating wavelength, and α is aproportionality factor for the twist angles, e.g. α=1.732 for twistangle=90°.

Thus, the modulator acts as a switchable half-wave plate that canselectably (in response to application or non-application of theelectrical field) rotate the input linear polarization by 0° or 90°.Without the electrical field, the twisted structure wave guides thepolarization of the input light to rotate the polarization by 90°. Withapplication of electrical fields, the waveguiding effect is distortedand the polarization is only partially rotated. With the modulatorsandwiched between two crossed or parallel polarizers, analog intensitymodulation can be obtained. In a TN geometry as described, the tiltangle of the molecules at the boundaries are perpendicular to eachother. This is believed to result in substantial cancellation of theresidual birefringence thus increasing contrast. Unfortunately,conventional TN modulators are inappropriate for many opticalcommunications applications because of relatively slow response times,being nearly an order of magnitude slower than parallel nematic LCmodulators.

Accordingly, previous materials and devices, while useful in manycontexts, including liquid crystal displays, have not previously beenconfigured to achieve both the high contrast and rapid switching speeddesirable for optical communications applications. For example, thematerial known as E44 available from E. Merck Industries has a responsetime of about 65 milliseconds when fabricated into 90° twistedstructures (e.g. for telecom applications). Such switching time can bereduced e.g. to less than about 10 milliseconds if a parallel cell isconstructed but such a parallel cell does not provide the necessarycontrast.

Another important factor that affects switching time is the thickness ofthe modulator. Switching time is roughly proportional to viscosity andinversely proportional to the square of the thickness: t∝γ/d². Thiswould indicate that faster switching is achieved with a low materialviscosity and a thinner cell. For optical communication applicationsoperating at infrared (IR) wavelengths (e.g. about 1550 nm) in order toobtain a thin cell, a large optical birefringence (on the order of 0.26)would be needed to maintain the thickness less than about 5 microns.

Accordingly, it would be advantageous to provide a device which achievesboth high contrast (such as an extinction ratio greater than about −25to −30 dB) and rapid switching (such as a recovery time of about 50milliseconds or less) preferably operating at infrared wavelengths andtemperatures in the range of about 20° C. to 40° C.

SUMMARY OF THE INVENTION

The present invention provides a hybrid analog/binary electro-opticalmodulator using a twisted nematic liquid crystal structure whichachieves both a high extinction ratio and rapid switching speed. Themodulator is configured with the relative rubbing direction for the twocell walls or the “twist angle” neither parallel, nor at 90° or180°-270°. Rather, the twisting angle is between 0° and 90°, preferablybetween about 50° and about 80°, more preferably between about 60° andabout 70°, to provide an acute twist nematic (hereinafter ATN) liquidcrystal device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C are schematic exploded perspective views showing relativepolarizer and buffing directions according to certain previous devices;

FIG. 2 is a schematic perspective exploded view of polarizer and cellwall components of a modulator according to an embodiment of the presentinvention showing relative polarization and buffing angles;

FIG. 2A is a side view of an ATN modulation device according to anembodiment of the present invention.

FIG. 3 is a graph depicting relative buffing directions for use in a 60°right hand twist modulator according to an embodiment of the presentinvention;

FIG. 4 is a graph depicting relative polarization directions and LCdirector directions for a modulator according to a embodiment of thepresent invention;

FIG. 5 is a graph comparing calculated transmission extinction ratioscalculated across a range of wavelengths for anti-parallel and 90° TNmodulators of previous design and a 60° TN modulator according to anembodiment of the present invention;

FIG. 6 is a graph depicting calculated transmission extinction ratiosacross a range of wavelengths for various values of beta angle;

FIG. 7A depicts transmission extinction ratios for a number ofwavelengths obtained using a parallel polarized modulator with 60° twistangle according to an embodiment of the present invention;

FIG. 7B is a graph corresponding to FIG. 7A but for a modulator withcross polarization;

FIG. 7C is a graph corresponding to FIG. 7B but for a device with a 70°twist angle;

FIG. 8 is a graph depicting optical response of a 60° twisted TNmodulator with parallel polarizers according to an embodiment of thepresent invention in response to an applied voltage signal wherein thehorizontal scale is graduated in divisions of 500 milliseconds and thevertical scale for the optical response is graduated in divisions of 200millivolts;

FIG. 9 shows a comparison of measured switching times for a 60° TN celland a 70° TN cell according to an embodiment of the present invention,at different operating temperatures;

FIG. 10 shows an optical network using an optical routing switch inaccordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Before describing embodiments of the present invention, certain aspectsof previous devices will first be described. As noted above, previous 0°and 90° structures commonly included entrance and exit polarizers andentrance and exit LC directors such as buffed cell walls. FIG. 1Adepicts relative directions for entrance and exit polarizers 112 a, 114a and entrance and exit LC directors 116 a, 118 a for previous parallelor 0° modulators. As can be seen from FIG. 1A, although the polarizationdirections of the entrance and exit polarizers 112 a, 114 a are crossed(i.e. orthogonal), the LC entrance and exit liquid crystal directors 116a, 118 a have identical directions such that the “twist angle” is 0°.Moreover, the entrance polarizer 112 a and entrance LC director 116 aare 45° offset so that the angle therebetween, referred to herein as the“beta” angle, is equal to 45°.

FIG. 1B depicts a structure for a 90° twisted nematic modulatoraccording to previous devices. As with FIG. 1A, the polarizers 112 b,114 b are crossed. In the configuration of FIG. 1B, the change inangular direction between the direction of the entrance LC director 116b and the exit LC director 118 b (measured in a right-hand direction122) is 90°, i.e. the configuration of FIG. 1B provides a 90° twistangle. As seen from FIG. 1B, the beta angle between the entrancepolarization direction 112 b and the entrance LC director 116 b is 0°.

FIG. 1C depicts angles found in a 270° “supertwist” device in which thetwist angle between the entrance LC director 116 c and the exit LCdirector 118 c (measured in a right-handed fashion) is 270° and the betaangle between the entrance polarizer 112 c and the entrance LC director116 c is 0°.

FIG. 2 depicts the relationship of angles in the device according to anembodiment of an ATN device of the present invention. FIG. 2 includesentrance and exit polarizers 212, 214 and entrance and exit LC directors216, 218. In the embodiment of FIG. 2, the entrance and exit polarizersdefine crossed polarization directions (i.e. polarization directions222, 224 that are orthogonal). The buffing directions or LC directionsof the entrance and exit LC directors 216, 218, however, define a twistangle (i.e. the angle which is passed-through, in a right-handed fashionwhen moving from the angle 226 of the entrance LC director 216 to theangle 228 or buffing direction of the exit LC director) is intermediatebetween 0° and 90°, preferably being between about 50° and 80°, morepreferably between 60° and about 70°. In the embodiment of FIG. 2, thebeta angle (i.e. the angle which is passed-through, in a right-handedfashion, when going from the orientation parallel to the polarizationdirection 222 of the entrance polarizer 212 to the buffing direction orLC direction 224 of the LC director 216) is greater than 0°, preferablybetween about 0° and about +25°, more preferably between about 5° and20°, more preferably between about 13° and about 17° for a 60° twistangle, and even more preferably, about 15°. For a 70° twist angle, thebeta angle is more preferably between about 8° and about 12°, even morepreferably, about 10°. According to one embodiment, it is believedsuperior transmission is obtained when β≈(90°−twist angle)/2. Forexample, in this embodiment, if the twist angle is 60°, preferablyβ=(90°−60°)/2=15°, and if the twist angle is 70°, preferably β=10°.

FIG. 2A is a side view of an ATN modulation device according to anembodiment of the present invention. The crossed entrance and exitpolarizers 212, 214 may be made from any of a number of well knownpolarizing materials, one example of which is that sold under the tradename POLARIZER, sold by Newport Optics of Irvine, California. Cell walls216, 218 may be formed of glass coated with transparent conductiveelectrode material such as indium-tin-oxide coating and with nylon orpolyimide layers which are buffed (e.g. with silk) to define buffingdirections and positioned to define angles as depicted in FIG. 2. Thecell walls with directors 216, 218 are positioned a distance apart 234such as about 5 microns apart and the space therebetween is filled witha liquid crystal material such as, for example, that available under thetrade name E44 from E. Merck Industries of the United Kingdom. Acontrollable voltage source 236 of a type well known in the art is usedto selectably apply voltage to the above described electrode layers toselectably switch the modulator between a substantially transmissiveIR-transmissive state and an IR-extinguishing state with a extinctionratio of greater than about −25 dB, preferably −30 or more dB and with aswitching speed or relaxation speed of about 50 milliseconds or less.

According to one embodiment of an ATN device as depicted in FIG. 3, thebuffing direction on the upper substrate 312 defines a twist angle 314of 60° with respect to the buffing direction 316 of the exit or lowersubstrate. Since the buffing directions are offset by 60° while thepolarizer directions are offset by 90°, there will be an angle betweenthe polarization directions and the entrance or exit LC directors (or,preferably, both). As depicted in FIG. 4, in one embodiment the betaangle 412 between the polarization direction 416 of the entrancepolarizer and the direction 418 defined by the entrance LC director isabout 15°.

An ATN modulator constructed as described in connection with FIGS. 2-4provides a transmission extinction ratio 512 across a bandwidth of lightas depicted in FIG. 5 intermediate between extinction ratio typicallyachieved in an anti-parallel modulator 514 and that typically achievedby a 90° TN-based modulator 516. As can be seen from FIG. 5, for a givenminimum or threshold extinction ratio (e.g. a minimum extinction ratioof −25 dB) the bandwidth 522 across which a 60° TN modulator according,to the present invention, is expected to operate is wider than theoperating bandwidth 524 for a 0° or anti-parallel device 514.

FIG. 6 depicts the fashion in which extinction ratio is calculated torelate to the beta angle of a 60° ATN device according to an embodimentof the present invention. As seen from FIG. 6, a device with a betaangle of 13° can achieve the extinction threshold of −25 dB over abandwidth 612 which is narrower than that 614 achieved by using a betaangle of 15°. Reducing the beta angle to 10° is expected to result in anextinction ratio 616 which never exceeds −20 dB. On the other hand, asbeta angles begin to exceed about 15°, the contrast or extinction ratioachieved by the device begins to deteriorate. The calculated sensitivityto beta angles, as depicted in FIG. 6, suggests that, for a deviceaccording to embodiments of the present invention, care in fabricatingdevices to achieve a desirable beta angle (such as about 15°, for a 60°ATN) will be rewarded.

As depicted in FIG. 7A, a 60° ATN modulator according to an embodimentof the present invention configured with parallel polarizers provides anextinction ratio 712, between −30 dB and about −43 dB across awavelength range from about 1525 nm to about 1575 nm. When a similar 60°TN device is provided with cross-polarizers according to the presentinvention, the extinction ratio 714 (FIG. 7B) across the same wavelengthrange is about −26 dB. By constructing a device in which the twist angleis increased to 70°, the extinction ratio 716 (FIG. 7C) can exceed about−30 dB 716. Thus it can be seen from FIGS. 7A-7C that an ATN deviceconstructed according to the present invention is able to achieve anextinction ratio of greater than −25 dB as desired.

FIGS. 8 and 9 show that an ATN device according to the present inventionis also able to achieve a switching time under 50 milliseconds asdesired. As shown on FIG. 8, when a change is made in an applied voltage812, a 60° ATN cell with parallel polarizers according to the presentinvention is able to achieve a desired optical response within a timeperiod 814 of about 10 milliseconds at 40° C. As shown in FIG. 9,switching time for both a 60° twist angle ATN modulator 912 and a 70°twist angle ATN modulator 914 is expected to display some temperaturedependence with both embodiments expected to achieve switching timesless than 50 milliseconds, preferably less than 35 milliseconds across atemperature range between 20° C. and 45° C.

ATN modulators as described above may, according to one embodiment ofthe present invention, be used in optical telecommunications systems. Asdepicted in the simplified diagram of FIG. 10, an optical network mayuse a modulator as described above as part of an optical routing switch1012 in selecting among paths e.g. 1014 a, 1014 b between a signalsource 1016 and destination 1018. By providing modulators havingrelatively high extinction ratios, a favorable signal-to-noise ratio inthe switch component 1012 for an optical network can be realized. Byproviding modulators which also achieve relatively high switching rates,it is possible to provide substantially uninterrupted service from thesource 1016 to the destination 1018 by permitting a control 1022 torapidly reconfigure the optical routing switch 1012 e.g. in response toa interruption on one of the paths (e.g. when an optical fiber is cut).

In light of the above description, a number of advantages of the presentinvention can be seen. The invention provides a useful and feasiblehybrid analog/binary electro-optic modulator using twisted nematicliquid crystal structure. The present invention provides for an ATNelectro-optic modulator which can simultaneously provide both a highextinction ratio (such as greater than −25 dB) and rapid switching (suchas less than 50-microsecond switching). Such simultaneous highextinction ratio and rapid switching speeds, while believed to be oflittle interest in connection with display devices, are of relativelygreat benefit in the context of optical communication application suchas optical modulators for optical routing switches in a fiber-optic orother optical communication network.

A number of variations and modifications of the invention can also beused. Although examples of liquid crystal and polarization materialsthat can be used in the context of the present invention have beendescribed, other materials are also operable. Although the invention hasbeen described in the context of optical routing switches, ATN devicesaccording to the present invention can also be applied to, e.g. displaydevices and high-speed spatial light modulators (SLMs). In general, the60° twist embodiment has a more rapid switching time than the 70° twistembodiment, as seen in FIG. 9, while the 70° twist embodiment has anextinction ratio that is about 5 dB larger than that of a corresponding60° twist embodiment.

Although the invention has been described by way of a preferredembodiment and certain variations and modifications, other variationsand modifications can also be used, the invention being defined by thefollowing claims.

What is claimed is:
 1. An optical communication system comprising:optical transmission media defining at least first and second opticaltransmission paths between a signal source and a signal destination; atleast a first routing switch coupled to said optical transmission mediato switch between said first and second transmission paths; said firstrouting switch including at least a first electro-optical modulatorwhich includes; first and second spaced apart cell walls; a liquidcrystal material positioned between said first and second spaced apartcell walls; electrodes positioned to impose an electric potential acrosssaid liquid crystal material; wherein said first and second cell wallsare configured to respectively define first and second liquid crystaldirector directions, said first and second directions being angularlyoffset by an amount defining a twist angle, wherein said twist angle isgreater than 0 degrees and less than 90 degrees, and wherein saidelectro-optical modulator has a switching time that is less than 50milliseconds.
 2. An optical communication system comprising: means fordefining at least first and second optical transmission paths between asignal source and a signal destination; at least a first routing switchfor switching between said first and second transmission paths; saidfirst routing switch including at least first and second spaced apartwalls means; a liquid crystal material positioned between said first andsecond spaced apart walls means; electrode means for imposing anelectric potential across said liquid crystal material; means fordefining first and second liquid crystal director directions, said firstand second directions being angularly offset by an amount defining atwist angle, wherein said twist angle is greater than 0 degrees and lessthan 90 degrees, and wherein said first routing switch has a switchingtime of 50 milliseconds.